In order to enhance security preventing a vehicle from being stolen, an immobilizer function is installed in a vehicle security system. In this system, an in-vehicle unit collates with a stored ID code an ID code acquired from a portable device such as a portable electronic key carried by a user by carrying out communication with the portable device, and controls locking/unlocking states of vehicle doors on the basis of a result of the collation.
More specifically, at a location in close proximity to the seat of a driver in a vehicle, a switch to be operated to start and stop an engine of the vehicle is provided. When the switch is operated to start the engine of the vehicle from a stopped state, the in-vehicle unit makes an attempt to communicate with the portable device and compares and collates the ID code acquired from the portable device with a stored ID code. When the result of the collation is OK, the in-vehicle unit outputs a start permission signal to an engine ECU. Receiving the start permission signal, the engine ECU becomes capable of controlling ignition timing and fuel injection quantity. Thus, the engine ECU is capable of starting the engine of the vehicle. When the result of the collation is NG, on the other hand, the engine of the vehicle cannot be started because no fuel and no ignition spark are supplied in the engine.
In such a system, however, the user is capable of unlocking a door, getting into the vehicle and starting the engine of the vehicle without having a portable device. It is thus possible that the degree of awareness of the portable device decreases. For this reason, it is likely that the driver is not aware of the fact that a passenger has taken out the portable device from the vehicle with the engine of the vehicle kept operated. In such a case, when the engine of the vehicle is once stopped, the engine can no longer be restarted because the portable device is not in the vehicle compartment.
In order to solve this problem, with the vehicle in a rest state and a door of the vehicle is opened and closed, for example, the in-vehicle unit makes an attempt to communicate with the portable device by using a vehicle compartment transmitter in order to verify that the portable device exists in the compartment. When the in-vehicle unit fails in the attempt to verify that the portable device exists in the compartment, the in-vehicle unit issues an alarm or warning in order to prevent the portable device from being taken out from the vehicle.
In this system, however, when a portable device is about to be taken out from a vehicle, an alarm is merely issued. Thus, it is possible that a passenger takes out the portable device mistakenly from the vehicle if the passenger is not aware of the alarm. In such a case, once the engine of the vehicle is stopped, the engine can no longer be restarted thereafter. U.S. Pat. No. 6,653,747 proposes an improvement to this problem.